This invention relates generally to feedback circuitry for charge digitizers, and more particularly to an improved feedback circuitry which will minimize transient voltages and also minimize the effect of variations in the input offset voltage of the integrating electrometer.
In the measurement of small currents or charges such as those encountered in ionization chambers, circuitry under the general title of charge digitizers or current to frequency converters is presently utilized so that the current or charge information can be treated digitally.
In this circuitry an input current charges an integrating capacitor until a following comparator is tripped which forces a monostable multivibrator to provide an output of rectangular voltage pulses. The voltage pulses cause similar current pulses in such a way that each current pulse is equal to a specific charge, and thus the pulse repetition rate is proportional to the input current.
In this type of charge digitizer, there is feedback circuitry to discharge the integrating capacitor, the feedback being initiated by the pulses of the multivibrator so that upon each pulsing of the multivibrator, the integrating capacitor is discharged. Conventionally this feedback circuitry has consisted of a resistor connected between the output of a multivibrator and the capacitor. This type of circuit has two major drawbacks.
First, the feedback circuit causes transient voltages at the edges of the feedback pulse which, while compensating each other, still can drive the integrator output beyond its dynamic range.
The second drawback, and by far the more serious, is that of variations in the input offset voltage of the electrometer, which can and will cause a false signal current of sufficient magnitude to adversely affect the output of the device and give false readings. Expressed another way, there is, in effect, a zero shift in the device which, for very small currents often involved in this type of device, is quite serious, and is especially pronounced when temperature varies.
While it is possible to reduce the erroneous current to some extent in the feedback circuit by utilizing a switch in the line which switch is normally open and closes only when there is a pulse current; nevertheless, this solution is not entirely satisfactory for general application since reed switches are too slow for general application and MOSFETS cause even larger transients which are extremely difficult to compensate for adequately.